We are attempting to purify a soluble form of adenylate cyclase present in the mature rat testis; though soluble, it is conceivable the primary structure of the soluble testis enzyme is similar to or even identical to the adenylate cyclase bound to membranes. Antibodies to purified test enzyme may interact, for example, with the hepatic adenylate cyclase and thus allow, with appropriately established methods quantitation of the amount of hepatic enzyme, perhaps localize the enzyme in its membrane form, and purify, on antibody affinity columns, the enzyme after solubilization. We have succeeded in purifying the testis enzyme 500-fold with the use of (NH)4)2SO4 precipitation, DEAE-argarose chromatography, Sephadex G-150 chromatography, ATP-agarose affinity columns. Hydroxy-apatite columns have recently given another 10-fold purification. Thus we are now in position to develop antibodies to the testis enzyme for the purposes outlined above.